Hi Mike S,
Part of the challenge of mattress design … as you know … is that much of it seems to be counterintuitive on the “surface”.
An example of this is the effect of layer thickness and how it works in combination with the softness of a layer and the thickness and softness of the layers above and below it (including the quilting/ticking). The layers are “interactive” with different people in different ways depending on sometimes small differences in weight distribution, body shape, and even small variances in sleeping positions which can create different very different weight distributions on a mattress.
In most cases … the relative depth of sinking between different parts of the body can create better or worse alignment rather than how much each part sinks in by itself. For example … a thicker 3" layer that has a slightly higher ILD and is “holding up” the wider lighter/greater surface area parts of the body but is still soft enough to have little effect on how deep the pelvis sinks (which will go through to the deeper layers anyway) can create misalignment. In cases like this … a softer layer on top can help the lighter parts with more surface area sink in a little more and create better alignment because the hips are mostly controlled by the firmer layer underneath that they come to rest on. This is especially true with foams that have a higher compression modulus.
The same foam that will “allow” one person to sink through it to different degrees may “stop” another person from sinking in enough in their lighter areas. Different “parts” of different people will either be “allowed” to sink or “stopped” from sinking at different levels of the same mattress. In effect … the same foam that can be “supportive” to one (the layer that they mostly come to rest on) and “pressure relieving” for another (a layer that compresses more deeply and goes through into the layers below). All of this of course is also determined by the “gaps” that need to be filled in and how much of the surface area of each person is taking on the load and relieving pressure from other more “protruding” areas. This is where the “art” of mattress construction takes over from the science.
It is usually very important to pay close attention to the interaction of layer thickness, layer ILD, and how deeply a certain part of a person is likely to sink into a mattress rather than each element by itself. Because of the infinite variables involved in body shape, weight distribution, and minor but important differences in sleeping positions … this is more part of the “art” of mattress construction.
In your case … I believe that a very soft extra layer will have little effect on your alignment and give some extra “cush” while for your wife who is lighter this same soft layer will help to isolate her from the firmness of the support layers which is likely the cause of her pressure.
Because you spend time on your stomach and this is in direct opposition of the needs of side sleeping … a thinner comfort layer makes sense to keep you closer to the support layers so any additional layers will need to be much softer to allow you to get to the support layers more easily.
The wool quilting in your mattress uses 1.5" of “loose” wool which is then compressed or densified into a thinner layer so it can act as a fire retardent. This too will play a role in how the mattress behaves.
So overall … my “gut” says a thin and very soft additional layer (softer than the 22 ILD that you eliminated) may well provide what both of you may need to move you closer to your ideal. Adding this on top will allow it to be softer while adding it into the cover will compress the upper layers more and increase the firmness slightly … and either may work well.
The only source of 1" of latex that I know in talalay (dunlop would be firmer) is Sleep Like a Bear. Other options could also be various other toppers that use either polyfoam or memory foam. These are widely available but some good sources are in the topper thread here. Thicker layers of wool that aren’t compressed and will provide more cushioning of the protruding parts rather than improve pressure relief “as a whole” throughout the whole surface of the body may also be a good choice but this is a more expensive option and while a well made thicker wool topper is quite resilient and will hold it’s softness for longer than other fibers, it will also eventually compress and become firmer.
Another option may be the “laNoodle” latex topper which is very soft and can be “re-fluffed” if the latex noodles shift. Because they more “flow” around you … they can provide extra softness without compromising support much like a very soft latex layer.
Lots of options … but it all seems to be more about “fine tuning” than it is about basic construction.